Yes!
And I found a solution!
Deleted the content of the Shaders Folder did the trick:
C:\Users\User\AppData\Local\Lockheed Martin\Prepar3D v2\Shaders\
They have been automatically been rebuilt starting P3D the next time ... and the effect is gone!
Other addons:
REX 4 Texture direct
REX Soft Clouds
ASN Next
Interesting find: The effect occurs also without rain.
And it is HDR Lighting connected. Turning HDR off, it's gone!